Technical Field
The present invention is directed to equipment for testing microcircuits.
Description of the Related Art
As microcircuits continually evolve to be smaller and more complex, the test equipment that tests the microcircuits also evolves. There is an ongoing effort to improve microcircuit test equipment, with improvements leading to an increase in reliability, an increase in throughput, and/or a decrease in expense.
Mounting a defective microcircuit on a circuit board is relatively costly. Installation usually involves soldering the microcircuit onto the circuit board. Once mounted on a circuit board, removing a microcircuit is problematic because the very act of melting the solder for a second time ruins the circuit board. Thus, if the microcircuit is defective, the circuit board itself is probably ruined as well, meaning that the entire value added to the circuit board at that point is lost. For all these reasons, a microcircuit is usually tested before installation on a circuit board.
Each microcircuit must be tested in a way that identifies all defective devices, but yet does not improperly identify good devices as defective. Either kind of error, if frequent, adds substantial overall cost to the circuit board manufacturing process, and can add retest costs for devices improperly identified as defective devices.
Microcircuit test equipment itself is quite complex. First of all, the test equipment must make accurate and low resistance temporary and non-destructive electrical contact with each of the closely spaced microcircuit probes. Because of the small size of microcircuit probes and the spacing between them, even small errors in making the probe will result in incorrect connections. Connections to the microcircuit that are misaligned or otherwise incorrect will cause the test equipment to identify the die under test (DUT) as defective, even though the reason for the failure is the defective electrical connection between the test equipment and the DUT rather than defects in the DUT itself.
A further problem in microcircuit test equipment arises in automated testing. Testing equipment may test 100 devices a minute, or even more. The sheer number of tests cause wear on the tester pins making electrical connections to the microcircuit terminals during testing. This wear dislodges conductive debris from both the tester pins and the DUT terminals that contaminates the testing equipment and the DUTs themselves.
The debris eventually results in poor electrical connections during testing and false indications that the DUT is defective. The debris adhering to the microcircuits may result in faulty assembly unless the debris is removed from the microcircuits. Removing debris adds cost and introduces another source of defects in the microcircuits themselves.
Other considerations exist as well. Inexpensive tester pins that perform well are advantageous. Minimizing the time required to replace them is also desirable, since test equipment is expensive. If the test equipment is off line for extended periods of normal maintenance, the cost of testing an individual microcircuit increases.
Test equipment in current use has an array of test probes that mimic the pattern of the microcircuit terminal array. The array of test probes is supported in a structure that precisely maintains the alignment of the probes relative to each other. A retainer and probe guide align the microcircuit itself with the test probes. The test probes, probe guide, and retainer are mounted on a probe card having conductive pads that make electrical connection to the test probes. The probe card pads are connected to circuit paths that carry the signals and power between the test equipment electronics and the test probes.
For the electrical tests, it is desired to form a temporary electrical connection between each terminal on the die under test and a corresponding electrical pad on a probe card. In general, it is impractical to solder and remove each electrical terminal on the microcircuit being contacted by a corresponding electrical probe on the tested. Instead of soldering and removing each terminal, the tester may employ a series of electrically conductive pins arranged in a pattern that corresponds to both the terminals on the die under test and the electrical pads on the probe card. When the die under test is forced into contact with the tester, the probes complete the circuits between respective die under test probes and corresponding probe card pads. After testing, when the die under test is released, the terminals separate from the probes and the circuits are broken.
The present application is directed to improvements to a probe array system capable of high performance testing for die with fine pitch.
There is a type of testing known as “Kelvin” testing, which measures the resistance between two terminals on the die under test. Basically, Kelvin testing involves forcing a current to flow between the two terminals, measuring the voltage difference between the two terminals, and using Ohm's Law to derive the resistance between the terminals, given as the voltage divided by the current. Each terminal on the die under test is electrically connected to two contact pads on the probe card. One of the two pads supplies a known current amount of current. The other pad is a high-impedance connection that acts as a voltmeter, which does not draw any significant amount of current. In other words, each terminal on the die under test that is to undergo Kelvin testing is simultaneously electrically connected to two pads on the probe card—one pad supplying a known amount of current and the other pad measuring a voltage and drawing an insignificant amount of current while doing so. The terminals are Kelvin tested two at a time, so that a single resistance measurement uses two terminals on the probe card and four contact pads.
In this application, the pins that form the temporary electrical connections between the die under test and the probe card may be used in several manners. In a “standard” test, each pin connects a particular terminal on the die under test to a particular pad on the probe card, with the terminals and pads being in a one-to-one relationship. For these standard tests, each terminal corresponds to exactly one pad, and each pad corresponds to exactly one terminal. In a “Kelvin” test, there are two pins contacting each terminal on the die under test, as described above. For these Kelvin tests, each terminal (on the die under test) corresponds to two pads (on the probe card), and each pad (on the probe card) corresponds to exactly one terminal (on the die under test). Although the testing scheme may vary, the mechanical structure and use of the probes is essentially the same, regardless of the testing scheme.
There are many aspects of the test beds that may be incorporated from older or existing test beds. For instance, much of the mechanical infrastructure and electrical circuitry may be used from existing test systems, and may be compatible with the electrically conductive probes disclosed herein. Such existing systems are listed and summarized below.
One particular type of microcircuit often tested before installation has a package or housing having what is commonly referred to as a ball grid array (BGA) terminal arrangement. A typical BGA package may have the form of a flat rectangular block, with typical sizes ranging from 5 mm to 40 mm on a side and 1 mm thick.
A typical microcircuit has a housing enclosing the actual circuitry. Signal and power (S&P) terminals are on one of the two larger, flat surfaces, of the housing. Typically, terminals occupy most of the area between the surface edges and any spacer or spacers. Note that in some cases, a spacer may be an encapsulated chip or a ground pad.
Each of the terminals may include a small, approximately spherical solder ball that firmly adheres to a lead from the internal circuitry penetrating surface, hence the term “ball grid array.” Each terminal and spacer projects a small distance away from the surface, with the terminals projecting farther from the surface than the spacers. During assembly, all terminals are simultaneously melted, and adhere to suitably located conductors previously formed on the circuit board.
The terminals themselves may be quite close to each other. Some have centerline spacings of as little as 0.1 mm, and even relatively widely spaced terminals may still be around 1.5 mm apart. Spacing between adjacent terminals is often referred to as “pitch.”
In addition to the factors mentioned above, BGA microcircuit testing involves additional factors.
First, in making the temporary contact with the ball terminals, the tester should not damage the S&P terminal surfaces that contact the circuit board, since such damage may affect the reliability of the solder joint for that terminal.
Second, the testing process is more accurate if the length of the conductors carrying the signals is kept short. An ideal test probe arrangement has short signal paths.
Third, solders commonly in use today for BGA terminals are mainly tin for environmental purposes. Tin-based solder alloys are likely to develop an oxide film on the outer surface that conducts poorly. Older solder alloys include substantial amounts of lead, which do not form oxide films. The test probes must be able to penetrate the oxide film present.
BGA test contacts currently known and used in the art employ spring pins made up of multiple pieces including a spring, a body and top and bottom plungers.
United States Patent Application Publication No. US 2003/0192181 A1, titled “Method of making an electronic contact” and published on Oct. 16, 2003, shows microelectronic contacts, such as flexible, tab-like, cantilever contacts, which are provided with asperities disposed in a regular pattern. Each asperity has a sharp feature at its tip remote from the surface of the contacts. As mating microelectronic elements are engaged with the contacts, a wiping action causes the sharp features of the asperities to scrape the mating element, so as to provide effective electrical interconnection and, optionally, effective metallurgical bonding between the contact and the mating element upon activation of a bonding material.